Image-forming materials useful for applications such as proof paper, print-out paper, overlay films and the like have widely been used in various photographic applications in which imagewise exposure and development provide an image in the photosensitive portion, known as free-radical photography.
The materials which are particularly useful as such image-forming materials are based on the radical-oxidative color development of a leuco dye by a change into a corresponding dye using a photo-oxidizing agent.
Some methods have been proposed to obtain a black-colored image with such photographic image-forming materials. For example, U.S. Pat. No. 4,078,934 discloses a method for obtaining a black color image by mixing two kinds of leuco dyes, using a biimidazole compound as a photo-oxidizing agent. Also, U.S. Pat. No. 4,379,835 discloses a mixed system of leuco dyes in which a nitrate is used as a photo-oxidizing agent. However, these methods have problems such as difficulty in hue adjustment due to color amalgamation, and the like.
U.S. Pat. Nos. 4,749,796 and 5,051,333 describe, as a method using a single leuco dye, a system which employs xanthene compounds having amino groups at the 2-position and the 6-position. However, it has been found that such image-forming materials develop color (fog) prior to use after long-term storage.